The present invention relates to knock detecting apparatus for detecting the presence of knock in an internal combustion engine.
Known apparatus of the above type are so designed that the vibrations caused in an engine by knocking are sensed by a piezoelectric type acceleration sensor employing a piezoelectric element.
However, the known apparatus of this type has the following disadvantages.
(1) Since the piezoelectric element used has essentially a high impedance, its characteristics tend to be affected by humidity and contamination and thus it is difficult to stably generate small trace knock signals (due to increased drift and vibration variations). Further, since the piezoelectric characteristic of the element is provided by polarization and since the apparatus is used by mounting it to an engine, that is, it is used under severe conditions which cyclically change from high temperature to low temperature conditions, the polarization tends to become gradually lower with a resulting decrease in the sensitivity. The decreased sensitivity results in an increased knock control level and thus it sometimes can cause damage to the engine.
(2) The detecting apparatus must preferably be constructed to reduce the cost as far as possible. However, piezoelectric elements have a high impedance and therefore it is necessary to use an expensive amplifier of the high impedance input type which is called a charge amplifier for amplifying the output signal of the piezoelectric element. Moreover, the high impedance type amplifier tends to malfunction under the effect, for example, of noise due to the ignition signals of the engine, thus requiring a more expensive and complicated construction.
(3) The use of the apparatus as a vehicle-mounted detecting apparatus is disadvantageous from the standpoint of durability, cost, etc., in that the piezoelectric element is inferior with respect to shock resistance, and is subject to cracking and breaking and further requires the use of such means as cooling when the apparatus is used at elevated temperatures.